Secret signaling



June 16, 1925.

R (3.. MATE-1E5 SECRET S IGNALING Filed June 30, 1923 a m H ,0? W Ev 5 wPatented June 1%, l gw like a ROBERT C. MATHES, OF NEVJ YORK, N. Y.,ASSIGNOR- T0 WESTERN ELECTRIC 601M- ?AN'Y, INCORPORATED, OF NEW YORK, N.'35., A GORPQRATIQN 0F NEW YQRK.

SECREC-i. SIGNALING.

Application filed June so, 1923. Serial No. 648,649,

To all whom it may concern:

Be it known that I, Bonner O. Marissa, a citizen of the .United States,residing at New York city, in the county of Bronx and State of New York,have invented certain new and useful Improvements in Secret Signaling,of which the following is a full, clear, concise, and exact description.

The present invention relates to the secret transmission of waves forany desired purpose, such as signaling;

The invention is particularly applhiahle to the transmissionandreception, with secrecy, of waves such as signals and the like comprisedof a considerable band of f equencies, as in the case of speech; Anobject of the invention is to obtain a high degree of secrecy withcomparatively simple terminal apparatus and by such transformations ofthe signal or other waves as will preserve a high degree of quality inthereproduced signals or waves at the receiver.

In the specific form of embodiment to be described herein, the speechwaves toJoe secretly transmitted are subdivided inti) "relatively narrowsub-bands of frequency components. A plurality of these sub-bandsnon-adjacent to one another in frequency are transmitted overyonetransmission path or channel, and other sub-bands are sent over adifferent channel. I For example, the sub-bands transmitted over thefirst channel. may comprise every alternate sub-band taken. in the orderof ascending frequencies, and the remaining sub-bands may be sent over adifferent channel. The frequency inter vals between the successivesubbands trans mitted over the same channel may be occupied by noisecomponents.

At the receiver arranged in accordance with the invention, the sub-bandsreceived over each path are separated from the noise currents bysuitable selective means and the sets of sub-bands transmitted over thedifierent channels are combined in the receiver. I

To an outsider who does not have access to all of the channels overwhich component pasts of the same message are transmitted, or who doesnot know the. number, width, and arrangement of the various sub-bands,cr has not the necessary filters to separate the speech components fromthe noise components, it is impossible to receive any inseptation of atransmitting station, and Fig.

2 is a similar representation of a receiving station adapted tocooperate with the station of Fig. 1 in carrying on communica tions withsecrecy.

he speech currents developed in circuit 2 as a result of speakingagainst transmitter l are applied to the series of subdividing o'ranalyzing filters F F F Each of these filters is a band transmissionfilter of theqtype disclosed in the patent to Campbell, No. 1,227,113 ofMay 525), 191? These filters are for the purpose of subdividing thenormal speech frequency band into sub-bands of relatively narrowfrequency ran e, each of which subhands by itsell is incapableolreproducing understandable speech,

,Tl'ie two sub-bands selectively transmitted by the respective filters Fand ll, nonadjacent in frequency, and are preferal'i y spaced apart infrequency by an interval equal to the width or the band trans sass bythe filter F Similarly, the iiiter F, transmits the next higher adjacentband to that transmitted by the filter F it being assumed. that thefilter F frequencies than the filter F as shown in this figure,therefore, it will be seen that the lowermost subband of the speechimpulse is transmitted by the filter transmits higher With thearrangement of the "four filters F The next higher sub-band. beginningsubstantially at the upper limit of the first I band is tra'nsmitted hy'the filter F Sinai-J larly, the next higher hand beginning substantially at the upper limit of the frequency range of the filter F istransmitted. by thefil ter F and the highest of the four sch-bands istransmitted by the filter E if more than four filters are used, thesub-bands may be of correspondingly narrower frequency range, and, as inthe case shown in the drawing', the analyzing filters will be groupedinto a plurality of sets, the filters of each set transmitting sub-bandswhich are non-ad jacent in frequency.

lU i

The sub-bands transmitted by the filters F and F are applied to themodulator M through the input transformer 3. This modulator is suppliedalso by a carrier wave from the source 4- so that the applied subbandsmodulate the carrier Wave, and produce for transmission a modulated Wavehaving two side bands, as is Well known in the art. ."Sources f and f ofcontinuous Waves are also arranged to be applied to the modulator Msimultaneously With the sub-bands from the filters F and F These wavesources are preferably vacuum tube oscillators of the type shown in thepatent to.

l lartiey, No. 1,356,763, October 26, 1920.,the tuning condensers 5 and6 of which are arranged to have their movable plates continuously drivenby the motor 7 so that the frequency of each generatorvaries cyclicallybetween two fixed limits. The limits between which the frequency of theoscillator f varies are of thesame order of frequency as the frequencylimits of the band transmitted by the filter-F Similarly, the frequencyof the oscillator f varies over a range equivalent to the transmissionrange of the filter F The rate of variation of the frequency of thesewave sourcesmay be sufiiciently high so that ifthey Were applied to atelephone receiver they would appear as noise currents havinga band offrequencies of the widthequivalent to the frequency separation of thelimiting frequencies bet-ween which they are varied.

As a result of applying to the modulator M both the sub-bands from thefilters F F and the noise currents from the oscillators f and f thecarrier wave 4 is modulated by a band of frequencies of normal speechfrequency width. It is preferred to use a balanced modulator of the typeindicated so that the unmodulat'ed carrier component is suppressed atthe transmitting station. Also by sharply tuning the trans mittingantenna TA, to one of the two side bands resulting from the modulation,the components that are transmitted can be re stricted to thosecomprising a single one of the two side bands. Instead. of using thetype of modulator shown, a modulator circuit of the type disclosed inthe patent to Osborne, No. 1,861,488, December 7', 1920, may be employedfor the puypose of restricting the transmitted components to thosecomprised in the single side band. By so restricting the transmittedcomponents, the actual amount of energy transmitted is only aboutone-sixth great would be the case if bothside bands and the unmodulatedcarrier omponcnt were transmitted. l v

The sub-bands transmitt d by the filters F, and F, are applied t themodulator M through the transforme- 8. This mod ulator is also suppliedwlth carrier waves; from the source 9 and with noise currents trom theoscillators f and 7". These oscillators have their tuning condensers 10and 11, respectively,continuously varied by the motor 7 so that theyfurnish noise currents composed of bands of frequency componentssubstantially equal in width to the transmis sion ranges of the filtersF and F respec having a side band width equal to the width of normalspeech and the unmodulated carrier component and one-of the two sidebands are suppressed as described in corinection with the modulator MThe remaining single side band is transmitted from the transmittingantenha TA Carrier wave sources 4 and 9 may each be of the typedisclosed in the Hartley patent; supra.

To an unauthorized person who intercepts either or both of the modulatedwaves from transmitting antenna TA, and transmitting antenna TA thereceived currents will appear only as noise currents incapable ofreproducing speech; This is true since the components of any singlesub-band represent only unintelligible components and it is impossibleto superpose the sub-bands transmitted. over both channels in such a wayas to build up the speech range, since the noise components transmittedfrom the sources f to f cover practically the entire speech range andmay readily be made of suclramplitude as to completely drown out thespeech components.

The speech transmitted by the system of Fig. l is received in accordancewitn the in vention by means of-an arrangement such as is shown in Fig.2. In this figure the receiving antenna RA, is tuned to receive themooulated wave transmitted. from the antenna TA, and the receivingantenna HA is tuned to receive the other of the two transmitted waves.

The frequency interval between the two transmitted waves should be largeenough to permit of good selectivity between the Waves at the receivingstation. relying on receiving circuits tuned to the transmitted wavefrequency, a beat oscillator may be used to step down the receivedfrequencies and the selection may be carried out at a frequency lowerthan the transmitted Wave frequency. II1'Bli1l1Q1 Ci1S8 a practicallycomplete separation will be made between the waves transmitted over oneof the high frequency paths or channels and those transmitted over theother channel, so that the waves from one of the channels are appliedexclusively through the tuned cir Instead of l if) aws? eating sir 1 UniLlu 0 D1. m 1 9 .h a a. J p 1 u .1 u m 0 W q, H T O4 C v 1 Wm A .i T Qtv hkmw w mwm m L rnmmmfiuwmuw Hum ww .3 run P ne L t T C a? T T. v .l.L Y a 0.... n 0 I 8.1.1CW? i. 2C 0 sbfific a O in 3 no 0 5 Au 5 A .1 55 the sub-bands from the several channels insecretly transmitted by themethod defined in claim 6, comprlsing separately detecting said carrierWaves to yield sub-bands of the frequencies used at the transmitter tomodulate the respective carrier Waves, selecting the sub-bands andapplying them in com- 10 mon to a receiving circuit.

11. In a telephone system, a source of speech currents, a plurality oftransmission channels, a plurality of frequency selec tive elements,each-selective of a different I frequencyconnected between each of thetransmission channels and "said source of speech currents.

In witness whereof, I hereunto subscribe my name this 26th'day of JuneA. D., 1923.

ROBERT C. MATHES.

